U.S. patent number 6,258,615 [Application Number 09/191,070] was granted by the patent office on 2001-07-10 for method of varying a characteristic of an optical vertical cavity structure formed by metalorganic vapor phase epitaxy.
This patent grant is currently assigned to Sandia Corporation. Invention is credited to Kent D. Choquette, Michael E. Coltrin, Hong Q. Hou.
United States Patent |
6,258,615 |
Hou , et al. |
July 10, 2001 |
Method of varying a characteristic of an optical vertical cavity
structure formed by metalorganic vapor phase epitaxy
Abstract
A process for forming an array of vertical cavity optical
resonant structures wherein the structures in the array have
different detection or emission wavelengths. The process uses
selective area growth (SAG) in conjunction with annular masks of
differing dimensions to control the thickness and chemical
composition of the materials in the optical cavities in conjunction
with a metalorganic vapor phase epitaxy (MOVPE) process to build
these arrays.
Inventors: |
Hou; Hong Q. (Albuquerque,
NM), Coltrin; Michael E. (Albuquerque, NM), Choquette;
Kent D. (Albuquerque, NM) |
Assignee: |
Sandia Corporation
(Albuquerque, NM)
|
Family
ID: |
22704016 |
Appl.
No.: |
09/191,070 |
Filed: |
November 12, 1998 |
Current U.S.
Class: |
438/35;
257/E21.131; 438/29; 438/31; 438/956 |
Current CPC
Class: |
H01S
5/0201 (20130101); H01S 5/423 (20130101); H01L
21/02463 (20130101); H01L 21/02546 (20130101); H01L
21/0262 (20130101); H01L 21/02639 (20130101); H01S
5/026 (20130101); H01S 5/18358 (20130101); H01S
5/2077 (20130101); H01S 5/4087 (20130101); H01S
2304/04 (20130101); Y10S 438/956 (20130101) |
Current International
Class: |
H01L
21/02 (20060101); H01L 21/20 (20060101); H01S
5/42 (20060101); H01S 5/00 (20060101); H01S
5/02 (20060101); H01S 5/40 (20060101); H01S
5/20 (20060101); H01S 5/183 (20060101); H01L
021/00 () |
Field of
Search: |
;438/29,35,956,31
;117/104 ;372/103,97 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Meier; Stephen D.
Assistant Examiner: Novacek; Christy L.
Attorney, Agent or Firm: Cone; Gregory A.
Government Interests
STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT
This invention was made with Government support under Contract
DE-AC04-94AL85000 awarded by the U.S. Department of Energy. The
Government has certain rights in the invention.
Claims
What is claimed is:
1. A method for forming at least two vertical optical resonant
cavity structures on a substrate with differing cavity
characteristics comprising:
providing a base surface suitable for metalorganic vapor phase
epitaxial deposition;
creating at least two different annular mask patterns on the base
surface wherein:
the mask patterns are formed from a dielectric material onto which
a set of semiconductor materials will not deposit, and
the inner diameters or the annular widths of the at least two
different mask patterns are different; and
depositing multiple layers of the set of semiconductor materials
onto the base surface to form the vertical resonant cavity
structures.
2. The method of claim 1 wherein the differing characteristic is
vertical thickness of the resonant optical cavities, the bandgap of
the cavities, the detection wavelengths of the cavities, the
emission wavelengths of the cavities, the chemical composition of
the deposited compound semiconductors in the cavities, or the
lateral variation of the chemical composition of within a cavity as
between different cavities.
3. The method of claim 1 additionally comprising forming a
distributed Bragg reflector mirror structure as the base surface
for subsequent deposition of the dielectric mask patterns
thereupon.
4. The method of the claim 1 wherein the resonant optical cavity
includes a quantum well active region.
5. The method of claim 1 wherein the dielectric material is
SiO.sub.2, SiN.sub.x, or SiON.sub.x.
6. The method of claim 1 wherein the annular masks define a
one-dimensional array of vertical cavity structures.
7. The method of claim 1 wherein the annular masks define a
two-dimensional array of vertical cavity structures.
8. The method of claim 1 wherein the set of semiconductor materials
are the III-V compound semiconductor materials.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
BACKGROUND OF THE INVENTION
This invention relates to methods of forming vertical resonant
cavity optical structures upon a substrate. More particularly, this
invention relates to a method for precisely varying at least one
characteristic of such structures between individual laterally
displaced structures on the substrate. The characteristics include
wavelength of emission or detection, bandgap energy, thickness of
layers within the structures, chemical composition of the layers
within the structures, and lateral variation of the amounts of
different elements within a layer.
The structures are vertical-cavity resonance optoelectronic devices
and include vertical-cavity surface-emitting lasers (VCSELs),
resonance-cavity photodetectors (RCPDs) and Fabry-Perot cavity
modulators (FPCMs) that are useful in optical communications and
sensing. A 1- or 2-dimensional (2D) device array emitting,
detecting, or modulating light at different wavelengths enables
many unique applications. For instance, a VCSEL array with
different wavelengths can be used for wavelength-division
multiplexing (WDM) fiber-optic communication systems. Different
wavelengths from different array elements can be coupled in a
single fiber for transmission over a distance. Each wavelength can
be differently encoded and a de-multiplexing system at the
receiving end can separate the different channels. Such a scheme
can greatly enhance the transmission capacity. A VCSEL array WDM
system is ideal for campus-wide short-haul applications. In
addition, such an array has been demonstrated to be very useful for
reconfigurable multiple chip module free-space interconnects. An
array of RCPDs sensitive to different wavelengths enables a compact
and integrated multi-channel spectroscopic analysis microsystem for
quantitative fast parallel optical sensing. The detectors consist
of a closely-spaced resonance-cavity detector array. Each element
in the array is only sensitive to a specific wavelength of the
broadband irradiation. Therefore, the array is equivalent to an
integration of a spectrometer and a detector array. An array of
FPCMs also offers a unique capability for multiple wavelength
communication systems.
The central part of the resonance cavity devices is the optical
cavity embedded between two distributed Bragg reflector (DBR)
mirrors. The wavelength of emission, detection, or modulation is
dictated by the Fabry-Perot mode of the cavity, which is determined
in turn by the optical thickness (the product of layer thickness
with the refractive index) of the cavity. The operating wavelength
will be changed if the thickness of the optical cavity is
changed.
It would be desirable to be able to fabricate an array of
resonant-cavity devices from a single growth with a reliable
manufacturing process. Such a process would require lateral
variation of the layer thickness and alloy composition in these
complex, multi-layer resonant structures. Prior to the invention of
the process disclosed herein, no such process was known to
exist.
Selective area growth (SAG) by metalorganic vapor phase epitaxy
(MOVPE) has proved to be a viable technique for the lateral
definition of the thickness, composition, and bandgap energy of
semiconductor material at different regions of the same wafer. The
substrate (or base epitaxial structure) can be partially masked by
dielectric materials, such as SiO.sub.2, SiN.sub.x, and SiON.sub.x,
and perfect selectivity of the growth (no growth occurs on the mask
material) can be achieved for many III-V materials under certain
growth conditions. The growth selectivity redistributes the flux of
reactant gases in the MOVPE growth, and diverts the metalorganic
materials from the mask region into the open area. Different
degrees of enhancement or modulation of the thickness and alloy
composition can be achieved on the same wafer nearby the different
mask patterns by varying the dimensions of the masked area. SAG
technique has been used for optoelectronic/photonic integration of
laser-modulators, multiple-wavelength (ID) laser-passive
waveguides, laser-detectors, and other similar linear stripe
structures. See for example, U.S. Pat. Nos. 5,704,975; 5,728,215;
5,770,466; and 5,828,085.
Review of the literature indicates that little has been done on the
demonstration of multiple wavelength RCPDs and FPCMs. However,
several approaches have been reported in making WDM VCSELs.
Chang-Hasnain et al reported a successful fabrication of a
multi-element VCSEL array with wavelength space .about.3 nm between
the neighboring elements in the array. C. J. Chang-Hasnain, J. P.
Harbison, C. E. Zah, M. W. Maeda, L. T. Florez, N. G. Stoffel, and
T. P. Lee, "Multiple wavelength tunable surface-emitting laser
arrays," IEEE J. Quant. Electron. vol. 27, pp. 1368, 1991. Their
approach was to use the inherent nonuniformity of the beam flux
profile in molecular beam epitaxy (MBE) growth. Substrate rotation,
which is used to average out the material nonuniformity, was
stopped during the growth of the cavity. Therefore, different
cavity wavelengths were achieved using the tapered cavity
thickness. Since the degree of the taper critically depends on the
substrate position relative to the sources, this approach has
problems with the producibility of the absolute wavelength and
wavelength spacing.
The second approach is to etch grooves on the backside of the
substrate for MBE growth. W. Yuen, G. S. Li, and D. J.
Chang-Hasnain, "Multiple-wavelength vertical-cavity
surface-emitting laser arrays with a record wavelength span," IEEE
Photon. Technol. Lett. vol. 8, pp. 4-7, 1996. The substrate is then
mounted to a Mo holder with In solders. The differential thermal
contact creates a temperature profile near the edge of the etched
groove. When the growth temperature is high enough, the material in
the cavity region creates a thickness profile due to differing
degrees of thermal desorption. Therefore, different wavelengths can
be achieved.
The third approach is to pattern and etch the substrate with
different sizes (and depths) of mesas. F. Koyama, T. Mukaihara, Y.
Hayashi, N. Ohnoki, N. Hatori and K. Iga, "Wavelength control of
vertical-cavity surface-emitting lasers by using nonplanar MOCVD,"
IEEE Photon Technol. Lett. vol. 7, pp. 10-12, 1995, and G. G.
Ortiz, J. Cheng, S. Z. Sun, H. Q. Hou, and B. E. Hammons,
"Monolithic, multiple wavelength vertical-cavity surface-emitting
laser arrays by surface-controlled MOCVD growth rate enhancement
and reduction," IEEE Photon. Technol. Lett. vol. 9, pp. 1066-1068,
1997. The topographical difference causes differences in the
surface diffusion process and, therefore, creates a cavity
thickness variation for different lasing wavelengths. Wavelength
difference was obtained from different mesas. This approach lacks
the accuracy of the wavelength control and introduces a
topographical profile, making it difficult to process. Another
major problem with these approaches is the narrow operable
temperature range. Since all the lasers with a large wavelength
span share the same active region of the quantum wells, the optimum
operating temperature for each element is very different. This
results in a very narrow temperature region of the quantum wells,
the optimum operating temperature for each element is very
different. This results in a very narrow temperature region within
which the operation of all the lasers can be achieved.
However, WDM VCSELs meeting the specification of the wavelength
accuracy, spacing, and device performance are becoming more
desirable for fiber-optic communications than ever before. For
example, a LAN project sponsored by the U.S. Government required a
1.times.4 array with wavelength spacing of 15 nm between the
neighboring channels. Currently, the laser array is rather
inelegantly achieved by using 4 discrete devices from 4
individually optimized runs. Clearly, an integrated device with
suitable performance would be superior if one could be made. For a
practical application of WDM VCSELs, the critical issues include a
predefined wavelength spacing, absolute wavelength accuracy for
each element, and uniformity of the device performance. The current
state of the art of technologies cannot satisfy these demands. It
would be very desirable if the gain wavelength for each individual
element of such a laser array "tracks" the laser wavelength so that
a uniform device performance over a range of wavelengths can be
achieved. None of the prior art above teaches a method to
successfully form these individual elements in 1 or 2 dimensional
arrays.
BRIEF SUMMARY OF THE INVENTION
The thickness and chemical composition and the resulting
wavelengths of emission or detection or bandgap of the active
region of a resonant cavity can be laterally varied across an array
of vertical cavity resonant structures by careful design of the
annular dielectric masks and selection of the precursor chemicals
used in the MOCVD technique. Use of the annular masks enables the
fabrication of the individual elements in the array. In cases where
the dielectric masks disrupt the needed selectivity of chemical
deposition of the layers in the distributed Bragg reflector layers,
the bottom DBR layers for the bottom mirror are formed without the
dielectric masks to create a uniform thickness DBR across the
substrate. The dielectric masks are then formed on top of the lower
DBR mirror structure, and the active regions of the various
individual resonant cavity structures are formed. The upper DBR
mirror is then formed on top of the individual active regions in
another uniform layer. The individual devices are later defined as
individual elements in an array by selective etching.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view diagram of a dielectric mask pattern, showing
a 4.times.4 array of annular masks having the same inner opening
diameter (120 .mu.m) but varying outer diameters. The structures
are spaced 1 mm apart to avoid interference therebetween.
FIG. 2 is a graph showing the variation of the thickness
enhancement factor as a function of the radius of an annular ring
for three different III-V compound semiconductor materials.
FIG. 3 is a graph showing the variation in thickness enhancement
factor and indium composition variation as a function of the width
of the annular ring. Nominal thickness and In composition on a
planar wafer are 1 and 0.18 respectively.
FIG. 4 is a graph showing wavelength variation (Fabry-Perot cavity
wavelength and modeled emission wavelength) as a function of
annular ring width. The variations result from enhancement of
thickness and In composition caused by varying ring mask width.
FIG. 5 is a graph showing thickness enhancement at the center of
the rings as a function of the width of the annular rings.
FIG. 6 is a schematic diagram of a wavelength-selective RCPD array
showing responsivity peaks at different wavelengths for elements
with different cavity thicknesses.
DETAILED DESCRIPTION OF THE INVENTION
One embodiment of the present invention is a 2-dimensional VCSEL
array that was built by applying the SAG technique to MOVPE. The
variation of the mask geometry (e.g., the mask area/shape or the
open area/shape) leads to a variation of the layer thickness and
alloy compositions. The variation of the mask geometry can be in
linear dimension or circumference (in the case of circular masks).
Here we consider an example of an annular pattern of doughnut
shape. Shown in FIG. 1 is a 4.times.4 array of the annular rings
with the same inner opening-area diameter of 120 .mu.m. The
difference in the width of the rings (masked area) leads to a
difference in the thickness of the material inside the ring. We
have used a two-dimensional steady-state finite-difference
calculation to model the flux of materials on the masked and
exposed areas. FIG. 2 shows the thickness profile in the open area
of the substrate for the different patterns calculated for InAs,
GaAs and In.sub.0.18 Ga.sub.0.82 As using trimethylgallium (TMG)
and trimethylindium (TMI) as the metalorganic sources. The growth
pressure and growth temperature were 60 torr and 720.degree. C.,
respectively, for this calculation. The thickness enhancement
factor is normalized to the growth thickness on a featureless
planar substrate. Since the source depletes almost exponentially
away from the edge of the mask, the thickness varies over the whole
radius of the feature. Use of the annular pattern helps to form a
flatter profile in the open area. When the source material diffuses
inward from the ring, although the amount of the source material
decays, the flux density of the material increases as the cross
section decreases when the radius is reduced. This effect can be
qualitatively demonstrated as follows:
exp(-ar)/2.pi.r.about.a.sub.1 +a.sub.2 r+a.sub.3 1/r+ . . . .
Compared with an exponentially decayed profile for linear mask,
this dependence on r produces a flatter profile of the thickness of
annular pattern.
As shown in FIG. 2, the GaAs and InAs thickness are enhanced by
very different amounts, being much steeper for InAs and GaAs, as a
result of the difference in diffusion constants of the TMG and TMI.
FIG. 3 shows the thickness enhancement factor and the In
composition variation of InAsAs in the middle of the ring. The
thickness variation was designed to be in a range of .about.8% so
that .about.70 nm of variation of cavity wavelength can be
achieved. Because of the difference in the diffusion rates of TMI
and TMG, the In composition also varies from 0.181 to 0.194. This
can be extremely useful in defining different emission wavelengths
for the gain of the quantum well active region.
FIG. 4 shows the calculated cavity wavelength variation due to the
thickness variation. The width of the mask was designed to achieve
a total variation of the lasing wavelength of 40 nm. (The stopband
width of GaAs/Al.sub.0.94 Ga.sub.0.06 As is usually .about.100 nm,
so all the wavelengths are still covered within the single DBR
structure). However, if the gain wavelength doesn't change with the
operating wavelength, the overlap of the gain with the cavity mode
can vary significantly for the different elements of the array. As
a result, the performance of the devices will be compromised, and
they will only be able to run in a narrow temperature range. For
the growth of an InGaAs quantum well, the In composition varies in
the center of the doughnut, as shown in FIG. 3, in addition to the
thickness. The variation in emission energy from the quantum wells
with different In compositions and well thickness are calculated
and plotted in FIG. 4 with the dotted line for the ring pattern.
The emission wavelength moves in the same direction as the cavity
wavelength modulated by the thickness variation, although they do
not change at exactly the same rates. With further optimization on
the mask geometry, we can obtain an even better "tracking" of the
two wavelengths. Therefore, the SAG by MOVPE provides not only a
variation of the cavity wavelength, but also a wavelength
"tracking" between the cavity mode and the emission wavelength to
ensure similar performances of the VCSELs in the same array over a
good temperature range. Because the diffusion constants of TMA and
TMG are about the same, a variation of the composition in AlGaAs
may not be achievable for AlGaAs which is important for 780 and
850-nm VCSELs. We can utilize strained InGaAlAs (as taught by J.
Ko, E. R. Hegblomer, Y. Akulova, J. J. Thibeault, L. A. Coldren,
"Low-threshold 840-nm laterally oxidized vertical-cavity lasers
using AlInGaAs-AlGaAs strained active layers," IEEE Photon.
Technol. Lett. vol. 9, pp. 863-865, 1997, incorporated herein by
reference) or InGaAsP active region for these wavelengths, or
AlGaAs grown with triethylgallium (TEG) and TMA since the diffusion
constants for these two precursors differ more.
We have grown InGaAs/GaAs and GaAs/AlGaAs quantum wells embedded in
AlGaAs cladding layers for the active region of VCSELs, and
performed thickness profile and photoluminescence (PL) measurements
for different doughnut dimensions. Perfectly selective growth on
the annular pattern was achieved on the dielectric mask. FIG. 5
shows the thickness in the middle of the ring for the growth of
InGaAs/GaAs quantum wells with AlGaAs cladding layers for 980-nm
VCSELs as a function of the mask width. The thick solid line
represents the model calculation for the thickness enhancement, and
the solid circles are measured thickness. The experimental results
are in an excellent agreement with the calculation. In order to
enhance the contrast of the DBR layers for mirrors, the low-index
material in the DBR is typically high Al composition AlGaAs or
AlAs. Unfortunately, it is extremely difficult to achieve a perfect
selectivity of AlAs on the dielectric mask. Therefore, a bottom DBR
is grown first, the sample is then unloaded for patterning of the
dielectric mask. This is followed by a SAG of the active region and
the top DBR can then be grown on atop. We have demonstrated that
the VCSEL performance from the selective area regrowth is almost
identical to that from a single growth. The remaining steps for
forming a VSCEL array are accomplished in a conventional manner,
i.e. vertical etching to form the individual VCSEL elements,
formation of electrodes, etc.
In order to achieve the wavelength variation and accuracy in a
controlled manner, the growth has to be extremely uniform. We have
demonstrated a uniformity of the laser cavity wavelength of better
than 0.5%, or 5 nm for 980 nm VCSELs. The wavelength accuracy can
be further improved by in situ control using normal-incidence
reflectance as the sensor (see W. G. Breiland, H. Q. Hou, H. C.
Chui, and B. E. Hammons, "In situ pregrowth calibration using
reflectance as a control strategy for MOCVD fabrication of device
structures," J. Cryst. Growth, vol. 174, pp. 564-568, 1997,
incorporated by reference herein). The simulation of the growth
rate enhancement in this work assumes that the crosstalk of the
gas-phase diffusion is negligible because the features are
separated far enough. This provides a valuable guideline for mask
design. A more complete case including arbitrary shape of the
dielectric mask pattern and boundary conditions can be accounted
for by a massive parallel numerical code, called MP-SALSA, and an
accurate determination of the enhancement for any shape of patterns
placed in any distance including the crosstalk effect can be
predicted.
In other embodiments of the present invention, two or more vertical
optical resonant cavity structures can be formed using annular mask
patterns with different inner diameters.
For the fabrication of the RCPDs, the responsivity can be very high
because of multiple paths through the active region. The spectral
width of the detection is determined by the Q of the cavity,
.DELTA..lambda.=.lambda./Q. For instance, .DELTA..lambda..about.1
nm for 980 nm wavelength if the Q of the cavity is .about.1000,
which is readily achievable with the DBR mirrors. Variation of the
resonance wavelength is again achieved by SAG. A schematic diagram
of the detector array is shown in FIG. 6. The tapered thickness is
produced here by a linear mask that is configured in a V shape with
equal widths on the arms of the V. The thickest portion of the
cavity forms at the narrow end of the V, and the cavity thickness
decreases as the V opens up. Alternatively, this structure could be
formed between parallel linear edges of two trapezoidal-shaped mask
strips. Here the thickest portion of the optical cavity would be
formed adjacent to the portions of the mask strips having the
greatest width. A combination of these two masking techniques could
also be employed. This densely packed detector array, sensitive to
only a narrow wavelength range for each element, can function as a
combination of bulky spectral dispersive device and a detector
array, will be very appealing for a compact sensing system
application.
Although the invention has been described in the context of the
VCSEL and RCPD structures set forth above, the true scope of the
invention is not limited thereto or the specific chemical elements
recited but is to be found in the appended claims.
* * * * *